1. Field of the Invention
The present invention relates to methods of cooling joints between superconductive cables such as used, for example, in magnets for magnetic resonance imaging (MRI) systems.
2. Description of the Prior Art
Joints of the above type are typically made by exposing the superconductive filaments within a superconducting cable, cleaning the filaments then braiding them together and infusing them with a superconductive alloy such as a Lead-Bismuth alloy PbBi. Typically, the joint is placed in a metallic cup which is filled with the PbBi alloy, to form the superconducting joint. Such action may be termed “potting” the joint. For such joints to remain superconducting, they must remain cooled to below the critical temperature of the filaments and the jointing alloy PbBi.
When used with conventional, bath-cooled magnet systems, maintenance of the required low operational temperature is straightforward, since the joints are immersed in boiling liquid helium and thus maintained at about 4·2 Kelvin. However, in other systems where the magnets are cooled by conduction, it is significantly more difficult to ensure that the joints do not assume temperatures higher than the critical temperature of the superconducting cables, as the joints cannot be immersed in a liquid helium bath or contained within a cold helium gas atmosphere. Furthermore, the joints are subjected to extremely high electrical voltages to ground, in the order of 5 kV, during quench events. It is accordingly necessary to provide an arrangement which will enable effective conduction cooling of the joints, yet provide adequate voltage isolation of the joints from other parts of the system.